The present invention relates to epoxy resins; to a process for preparing said resins and to compositions containing these resins.
Due to their physical and chemical properties such as resistance to chemical attack, good adhesion to various substrates, solvent resistance and hardness, epoxy resins are useful in a wide variety of commercial applications including the coating of various substrates such as metal, wood and plastic, and the preparation of structural and electrical laminates. In many applications such as the coating of the interior of containers ("cans"), the epoxy resin is applied from an organic liquid solution or aqueous dispersions.
Epoxy resins of differing molecular weight (so-called "advanced epoxy resins") can be prepared by the reaction of a polyepoxide such as the diglycidyl ether of bisphenol A with a polyhydric phenol such as bisphenol A.
The molecular weight of the epoxy resin generally affects the softening point, melt viscosity and solution viscosity of the epoxy resin as well as the physical and chemical properties of the cured product prepared therefrom. It is often desirable to prepare as high a molecular weight epoxy resin as practical to provide a product of sufficient toughness. High molecular weight resins are generally prepared by a two-step process wherein a lower molecular weight epoxy resin is prepared initially by reacting a polyhydric phenol with epichlorohydrin and alkali metal hydroxide in the presence of a catalyst. Thereafter, the initial polyepoxide reaction product is advanced by its reaction with additional amounts of polyhydric phenol to form the higher molecular weight material. In conventional techniques for preparing the epoxy resins, the reaction of the polyepoxide and polyhydric phenol is typically carried to complete conversion such that the final, advanced epoxy resin contains relatively low amounts of residual phenolic hydroxyl groups. For example, epoxy resins having an EEW (epoxy equivalent weight) between about 500 and about 700 prepared from bisphenol A and the diglycidyl ether of bisphenol A typically contain less than about 800 parts per million of phenolic hydroxyl groups which represents more than about 98 percent conversion of the phenolic hydroxyl groups employed in preparing the epoxy resin. A higher molecular weight epoxy resin having an EEW from greater than about 2000 to about 4000 typically contains less than about 2500 ppm of phenolic OH groups which represents more than about 95 percent conversion of the phenolic hydroxyl groups. Any residual hydroxyl groups in the advanced resin have been stated to cause viscosity instability of the resulting resin mixture, particularly at elevated temperatures. As a means for controlling the stability of the resin due to the unreacted phenolic hydroxyl groups, U.S. Pat. No. 3,842,037 suggests adding a strong, inorganic acid when at least about 85, more preferably at least about 95, percent of the phenolic hydroxyl groups employed in the advancement reaction have been reacted.
Alternatively, in another method for preparing a high molecular weight epoxy resin, U.S. Pat. No. 3,352,825 teaches condensing a dihydric phenol with an excess of epichlorohydrin in the presence of a catalyst such as an alkali metal or ammonium salt of an inorganic monobasic acid to form an intermediate having a free hydroxyl content in the range of from about 0.2 to about 0.95 phenolic hydroxyl group per mole of said dihydric phenol. Subsequently, the excess epichlorohydrin is removed and the intermediate condensate subsequently dehydrochlorinated, using caustic alkali and simultaneously the free phenolic hydroxyl groups are reacted with the epoxy groups formed in situ to form a product free of hydroxy groups.
Unfortunately, increasing the molecular weight of an epoxy resin also generally increases the melt and solution viscosities of the resin. Such increase in melt and solution viscosities renders the application of the epoxy resin more difficult.
One method by which the melt and solution viscosities of an epoxy resin can be reduced for a given EEW is by regulating the chain growth using a monofunctional reactant such as a monofunctional phenolic or epoxy compound as a capping agent. Unfortunately, the use of these capping agents results in a formation of an epoxy resin having reduced epoxy functionality and a lower softening point at a given EEW. The reduction in epoxy functionality markedly reduced the physical properties such as toughness of the cured resin product prepared therefrom.
In view of the aforementioned characteristics of the epoxy resins known in the prior art, it would be highly desirable to provide an epoxy resin having a lower melt and/or solution viscosity without a coincident and significant decrease in the softening point of the resin or in the physical properties of the resulting products prepared from the resin.